Nowadays, rain sensors are widely used for the automatic control of windshield wipers in motor vehicles. Such a rain sensor is based on, e.g., an optoelectronic measurement by means of a light emitting diode (LED) as a light source and a detecting photodiode, wherein the physical laws of reflection at the interface between a material with a higher refractive index and a material with a lower refractive index are made use of.
There are also other operational principles for realizing a rain sensor. For example, the change of the capacitance of a rain-moistened electric capacitor may be used.
Furthermore, systems that generate images of the windshield of a vehicle by means of a camera are used, wherein image processing systems evaluate said images with regard to raindrops on the outer surface of the windshield or with regard to condensate on the inner surface of the windshield.
For example, WO 2004/007255 A2, which is incorporated by reference, describes a system with a video camera. By means of said video camera, moisture on a windshield of a vehicle can be detected. To this end, at least one image is acquired after focusing the optical system of the video camera on the region of the windshield surface. The image data of said image are analyzed for moisture on the assumption that raindrops can be identified as circular objects in the image data, whereas precipitation on the outer surface of the windshield caused by fog can be detected on the basis of the scattering of light coming from outside the vehicle.
DE 102004037871 B4, which is incorporated by reference, shows a camera system for an assistance system, said assistance system covering the region outside a motor vehicle in the direction of motion of the motor vehicle, i.e., the region in front of the motor vehicle. The far range (road scene) and the near range (windshield) are imaged onto an image sensor by installing a front lens in a part of the field of view in front of the objective lens of the vehicle camera. DE 102004037871 B4 proposes the use of an image sensor for an outside-region assistance function and rain functionality.
Furthermore, DE 698 36 344 T2, which is incorporated by reference, describes a rain sensor system with a camera, which system enables condensate on the inner surface of the windshield to be detected by means of an additional light source so that a windshield heater is switched on instead of a windshield wiper, for example.
For rain detection, according to DE 10 2006 016 774 A1, which is incorporated by reference, a camera focused to infinity is used and the image data are evaluated on the basis of a change of pixel intensities contained in the image relative to an average pixel intensity.
WO 2006/024247 A1, which is incorporated by reference, too describes a method for the detection of precipitation on a windshield, wherein a camera is focused to infinity so that an out-of-focus image of the windshield is formed. Raindrops or condensate on the inner surface of the windshield cause a soft-focus effect. The definition of the image and/or the contrast differences between adjacent pixels are assessed, and the presence of precipitation on the windshield is inferred therefrom. Furthermore, for a situation in which the surroundings of the vehicle is poor in contrast, said publication proposes acquiring a first image in a first step, activating a windshield wiper or a windshield heater in a second step, and acquiring a second image in a third step in order to assess the two images with regard to changes.
Finally, fog detection systems that are used to control fog lamps and rear fog lamps are also known. For example, according to DE 695 04 761 T2, which is incorporated by reference, an image of the surroundings is acquired by means of a camera, and the image data of said image are evaluated on the basis of a contrast analysis for the detection of fog.